Add IndexSet::shift_insert based on map's method

src/set.rs

@@ -353,6 +353,20 @@ where
         (index, existing.is_none())
     }
 
+    /// Insert the value into the set at the given index.
+    ///
+    /// If an equivalent item already exists in the set, it returns
+    /// `false` leaving the original value in the set, but moving it to
+    /// the new position in the set. Otherwise, it inserts the new
+    /// item at the given index and returns `true`.
+    ///
+    /// ***Panics*** if `index` is out of bounds.
+    ///
+    /// Computes in **O(n)** time (average).
+    pub fn shift_insert(&mut self, index: usize, value: T) -> bool {
+        self.map.shift_insert(index, value, ()).is_none()
+    }
+
     /// Adds a value to the set, replacing the existing value, if any, that is
     /// equal to the given one, without altering its insertion order. Returns
     /// the replaced value.

src/set/tests.rs

@@ -127,6 +127,25 @@ fn insert_order() {
     }
 }
 
+#[test]
+fn shift_insert() {
+    let insert = [0, 4, 2, 12, 8, 7, 11, 5, 3, 17, 19, 22, 23];
+    let mut set = IndexSet::new();
+
+    for &elt in &insert {
+        set.shift_insert(0, elt);
+    }
+
+    assert_eq!(set.iter().count(), set.len());
+    assert_eq!(set.iter().count(), insert.len());
+    for (a, b) in insert.iter().rev().zip(set.iter()) {
+        assert_eq!(a, b);
+    }
+    for (i, v) in (0..insert.len()).zip(set.iter()) {
+        assert_eq!(set.get_index(i).unwrap(), v);
+    }
+}
+
 #[test]
 fn replace() {
     let replace = [0, 4, 2, 12, 8, 7, 11, 5];